Robert L. McKenzie and Douglas G. Fletcher, Ames Research Center, .... that band, absorbing transitions may be accessed in the 02 B3Iu
NASA Technical Memorandum 103928
*
P. 73
Laser-Spectroscopic Measurement Techniques for Hypersonic, Turbulent Wind Tunnel Flows Robert L McKenzie and Douglas G. Fletcher
March 1992
(NASA-TM-103928) LASERrSPECTROSCOPIC MEASUREMENT TECHNIQUES FOR HYPERSONIC TURBULENT WIND TUNNEL FLOWS (NASA) 13 p CSCL 148 G3/35
NASA
National Aeronautics and Space Administration
Unclas 0077064
NASA Technical Memorandum 103928
Laser-Spectroscopic Measurement Techniques for Hypersonic, Turbulent Wind Tunnel Flows Robert L. McKenzie and Douglas G. Fletcher, Ames Research Center, Moffett Field, California
March 1992
NASA
National Aeronautics and Space Administration
Ames Research Center Moffett Field, California 94035-1000
LASER-SPECTROSCOPIC MEASUREMENT TECHNIQUES FOR HYPERSONIC, TURBULENT WIND TUNNEL FLOWS
ROBERT L. MCKENZffi AND DOUGLAS G. FLETCHER NASA Ames Research Center Moffett Field, California, USA
1. Introduction Until recently, aerodynamic measurements obtained away from a surface have been limited either to the use of intrusive probes to determine pressure, mass flux, and total temperature or to visualization methods for indications of the flow geometry. New laser-spectroscopic methods are now emerging which offer compelling opportunitiesto obtain simultaneous measurements of density, temperature, and their fluctuations, with unprecedented spatial and temporal resolution [1]. As a result, measurement techniques are now feasible which can be used in supersonic and hypersonic flows to characterize the effects of compressibility on turbulent behavior and to obtain quantitative descriptions of complex, three-dimensional flow fields. Most of the laser-spectroscopic techniques incorporate laser-induced fluorescence (LIF) or Raman spectroscopy. The primary objective of this report is to describe the spectroscopic nature, present status, and capabilities of some LIF and Raman techniques which have been developed at NASA Ames Research Center. Particular emphasis has been centered on applications in hypersonic wind tunnels where measurements with more conventional probes are often inadequate or impractical. The requirements for laser-spectroscopic measurements in hypersonic flows are dictated by their intended aerodynamic research applications. By design, hypersonic flow fields over test articles arc generally free of large stagnation regions. Consequently, in moderately heated facilities, they can have densities lower than 0.01 amagat and temperatures ranging from SO K to 300 K. To provide sufficient sensitivity at the low temperatures, the spectroscopy must be based on individual rotational transitions in the interacting species. In addition, most applications are generally limited to chemically stable species which have been seeded into the flow or which occur naturally in air. With these requirements in mind, two principal laser-spectroscopic approaches have been developed at NASA Ames Research Center for the measurement of temperature, density, and their fluctuations owing to turbulence in high-speed flows. Early work was centered on the use of near-ultraviolet lasers to induce fluorescence from trace concentrations of nitric oxide (NO) added to flows of nitrogen. More recently, advances in laser technology have allowed similar measurements to be made in unseeded air flows using vacuum-ultraviolet lasers to excite molecular oxygen.
2. Laser-Induced Fluorescence in Flows Containing Nitric Oxide Prior to the availability of high-energy excimer lasers, a spectroscopic approach was developed which relied on the existing Nd:YAG-pumped, tunable dye laser technology. It was based a dualfrequency LIF technique in which two frequency-doubled dye lasers were each tuned to a nearultraviolet, ro-vibronic transition in the Gamma Band spectrum of nitric oxide [2]. Pulse energies of 1 uJ could be achieved over the entire electronic band near 225 run. The two independent, coincident LIF signals allowed density and temperature to be determined simultaneously. NO was added in concentrations of less than 100 ppm to a wind tunnel reservoir containing pure nitrogen. Because the NO fluorescence is quenched efficiently by oxygen and very little by nitrogen, the technique is most applicable in pure nitrogen flows. The capabilities of the method were demonstrated in the turbulent boundary layer on the wall of a Mach-2 channel flow. Single-pulse measurements were obtained with a spatial resolution of less than 1 mm and uncertainties of less than 2%. A small blow-down wind tunnel was used which was designed to handle gas mixtures containing NO. Since NO is toxic in concentrations greater than 25 ppm, a test facility suitable for NO-LIF must incorporate all of the special features necessary to safely use toxic mixtures. Despite this inconvenience for general applications, the NO-LIF technique demonstrated the unique capabilities of laser-spectroscopic methods for aerodynamic measurements. Its success encouraged investigations of similar techniques which could be applied to unseeded air flows. 3. Raman Scattering and Laser-Induced Fluorescence in Air Flows 3.1. BACKGROUND
The development of high energy, vacuum-ultraviolet, ArF excimer lasers made feasible the use of LIF from oxygen molecules in air. Exploration of the concept was initiated in 1981, before such lasers were available commercially, with a study solicited by NASA-Ames and performed by Massey and Lemon [3]. They were able to construct a tunable, ArF laser and demonstrate its use to obtain 02 LIF signals in room air. Their measurements indicated that the LIF was sufficiently intense and sensitive to temperature to be a viable measurement technique in unseeded air flows. The results of Massey and Lemon and the subsequent commercial availability of high-energy ArF excimer lasers encouraged further development of the concept. However, unlike the twolaser system used for NO-LIF, the cost and complexity of excimer lasers made the use of two lasers unattractive. Alternatively, Gross and McKenzie [4] proposed that both the fluorescence emission and the Raman scattering from each pulse of one laser be used to provide the two independent signals required for simultaneous temperature and density measurements. Since then, Laufer et al. have characterized the spectroscopy of 02-LIF [5] and demonstrated its use to measure temperatures down to 130 K in a cell at known pressures [6]. The development was continued by Fletcher who incorporated the Raman signal to obtain simultaneous measurements of temperatures and densities, without knowledge of any other thermodynamic parameters [7]. Most recently, he has demonstrated the use of the 02-LIF/Raman technique to measure temperature, density, and their fluctuations in the turbulent boundary layer of the same Mach-2 channel flow which was studied previously using NO-LIF [8].
3.2. 02-LIF/RAMAN SPECTROSCOPY
The 02 LIF process is characterized by the energy-level diagram shown in Fig. 1. The tuning range of the ArF laser is centered at 193.3 nm with a bandwidth of approximately 0.5 nm. Within that band, absorbing transitions may be accessed in the 02 B3Iu
